This invention relates to plastics injection nozzles.
For the injection of plastics material, nozzles are used which comprise a movable closure device guided in a radial extension of the body and acted on to close the orifice of the injection nose by an elastic element disposed concentrically to the exterior of the body of the nozzle.
Such a type of construction enables the spring to be protected from the direct effect of the temperature of the plastics material in the plastic state, and consequently to avoid or to reduce the known weakening effect. Such a construction also enables a good axial and radial guidance of the closure device which can then be constructed with a cylindrical projection at the end forming a piston penetrating it into the cylindrical injection bore of the nose at the end of the injection phase to provide a secondary pressure.
Nevertheless, it is to be noted that nozzles of this type have two major disadvantages, that is to say, the relatively large amount of space used due to the pressure of the external spring and the existence between the guide extension and the nose, of a retentive zone or cavity or to there being less circulation of the material which thus undergoes carbonization prejudicial to the good quality of the items produced.
There are also known nozzles comprising an automatic closure element guided axially in an internal support body containing a control spring for closing the closure device. In general, the internal body relies on the reaction of the spring of the closure device on the guide seat to isolate between two phases of injection, the feed circuit and the transfer and pressure build up chamber. Such a construction has the advantage of substantially reducing the space occupied thereby and thus the external diameter of the nozzle but, on the other hand, has an important disadvantage due to the fact that in the open position, the internal body is no longer accurately guided. Consequently it becomes impossible in these constructions to make the needle in the form of a cylindrical extension intended to penetrate the injection orifice of the nose.